The present invention deals with protection of a transmission against unauthorized reception.
There are three ways to protect a transmission against unauthorized reception: changing data to be transmitted, changing signals and changing channels. The first way is known as encrypton, the second and the third are utilized in spread-spectrum radio communication. The two main spread-spectrum techniques are direct-sequence spectrum spreading and frequency hopping. In frequency hopping, the transmitter repeatedly changes the center or carrier frequency, which is a radio channel. All three ways of protecting communications often involve generation of a pseudo-random succession, that appears random while it is determined by a secret key. That makes a difference between an intended receiver provided with the key, and unintended one. A problem of frequency hopping is interference harmful for quality communication, especially for television and computers.
This invention relates to protection of multichannel systems such as telephone, cable TV, computer and multimedia networks. The systems may divide channels using frequency-division, time-division, code-division or wavelength-division, and transmit analog or digital signals by land using telephone cables or fiber optics, or through the atmosphere using microwave relay stations or satellite. The method of this invention consists in frequent changing of the distribution of transmissions on channels with a switch controlled by a pseudo-random generator. As a result, every transmission is wandering from channel to channel in a pseudo-random manner. This way of protection does not require additional channels and completely excludes mutual interference of transmissions. The method ensures a reliable protection by itself and gives a synergetic effect in combination with encryption because it makes the transmitted ciphertext inaccessible for attack.